A biopsy is a medical test involving the removal of cells or tissues from a region of interest such as a lesion for medical examination purposes. The removed cells or tissues may be examined in order to detect the presence or extent of a disease.
For removing the cells or tissues, a biopsy needle has to be introduced into a patient's body and has to be guided to the region of interest. In order to enable monitoring of the introduction and guidance of the biopsy needle, ultrasound transducers are frequently used to observe the region of interest during the insertion of the biopsy needle.
As described for example in WO2006/060657-A2, a biopsy needle guide adapted for guiding a biopsy needle along a biopsy path may be coupled to a conventional ultrasound transducer device.
As schematically shown in FIGS. 1-3, a conventional ultrasound transducer 103 comprises a one-dimensional (1D) transducer face 104 from which ultrasonic signals may be emitted. Thereby, an image plane underneath the transducer face may be observed by detecting echoes of reflected ultrasonic signals coming from inhomogeneities of the observed region. By correctly positioning the ultrasound transducer 103, a region of interest comprising for example a lesion 108 may be observed. A biopsy needle guide 105 may be attached close to the transducer face 104 and may be adapted to guide a biopsy needle along a biopsy path 107 into the lesion 108.
Conventional one-dimensional ultrasound transducers comprise an array of elements 109 arranged in a line. A division of elements 109 in a line allows each element 109 to transmit and receive separate ultrasound signals, which may be combined to generate an image. The transducers array face 104 is usually a rectangle, where the long direction is generally referred to as the “azimuth” direction and the orthogonal direction is generally referred to as the “elevation” direction. Because the elements 109 are arranged in a single line, the ultrasound beam can be steered and focused in a region that is orthogonal to the face 104 of the transducer 103 but which is most simply described as a plane. This plane extends in the azimuth direction and in the “range” direction, where the range direction is orthogonal to the transducer array face and therefore orthogonal to both the azimuth and elevation directions. Although the transducer face is usually a rectangle, the field of view may be a triangle, rectangle or trapezoid that is orthogonal to the array face 104 and extends in the azimuth and range directions; this is generally referred to as the azimuthal plane.
In the example shown in FIG. 2, the field of view 110 is trapezoidal. The length of the transducer array in the elevation direction may determine, in conjunction with a mechanical lens, the focal characteristics in the elevation direction generally referred to as “slice thickness”. Ideally, slice thickness would be zero so that the image represents a cross-section of the patient orthogonal to the face of the array, but in practice, slice thickness cannot be zero. The image presented on the ultrasound system screen, while portrayed in a single plane, is in fact a projection of the ultrasound information contained in the non-zero slice in the azimuth plane. The slice thickness is not constant throughout the depth of the image. At the face of the transducer, it is equal to the elevation direction; as depth increases the slice thickness decreases as the elevation direction and lens curvature are combined to focus the ultrasound energy; past the focal depth, i.e. the depth at which the slice thickness is smallest, the ultrasound beam diverges and the slice thickness increases. To further complicate matters, the ultrasound beam does not have perfectly sharp boundaries. In the below description, simple planar structures will be used to illustrate ultrasound imaging fields of view that are in fact a fairly complex sampling of a three-dimensional space.
While in FIG. 1, the ultrasound transducer 103 is shown with a planar one-dimensional array face 104, the one-dimensional array can also be formed on a curved surface, in which case it is generally referred to as a “curved linear array” (CLA). This is illustrated in FIG. 3. Such a CLA, rather than being a rectangle, is a section of a cylinder. The resulting field of view 110′ is a section of a circle with the inner boundary being the array face 104′. Although, in the following description, most of the examples presented are for a flat array, the ideas of the invention may be equally applicable to a curved array.
As in conventional biopsy guide systems the ultrasound transducer acquires an image of the observed tissue only along an azimuthal plane, the biopsy needle guide attached to the ultrasound transducer is usually designed to support needle entry on the azimuthal plane. A different needle guide location could be used, but since the biopsy needles path would not fall within the single available imaging plane, no guidance imaging may be available as the needle will not be visible until it passes through the imaging plane.
Accordingly, in conventional biopsy procedures, the biopsy guide system including the ultrasound transducer will be moved along the surface above the region of interest until the actual region of interest (including e.g. a lesion) may be seen on the acquired ultrasound image, i.e. until the imaging plane crosses the region of interest. Then, the biopsy needle may be introduced into the tissue along the azimuthal plane. As the needle moves along the imaging plane, the current location of the needle may be monitored in the ultrasound image. A biopsy may be taken as soon as the region of interest has been reached.